The present invention generally relates to a method for removing foreign matter using a supercritical or subcritical medium, and more particularly relates to a method for forming a dielectric film such as a silicon dioxide film or ferroelectric film, a semiconductor device and a film deposition apparatus.
As the quantity of data processable per unit time by present-day high-performance electronic units goes on increasing, demand for large-capacity memories with a capacitive insulating film, like dynamic random access memories (DRAMs), is escalating. However, increase in area occupied by memories on a chip should be minimized to meet the downsizing requirement imposed on the electronic units of today. Thus, to realize a large-capacity memory while avoiding the increase in occupied area, capacitance per unit area of an insulating material for the capacitive insulating film should be increased to such a level that the memory does operate normally even if the area of a unit cell is reduced. For that purpose, capacitive insulating films made of ferroelectric or high-dielectric-constant materials have been applied recently.
Examples of high-dielectric-constant materials include Baxe2x80x94Srxe2x80x94Tixe2x80x94O (BST) and Pbxe2x80x94Zrxe2x80x94Tixe2x80x94O (PZT). When a high-dielectric-constant material is used, the stereoscopic configuration of a cell should be modified in some way or other to realize a DRAM with a capacity on the order of at least 1 gigabits and preferably more. For example, the area of the capacitor cell can be reduced and yet a sufficient quantity of charge can be stored in a unit area if tiny stepped portions are provided. Ferroelectric materials such as Srxe2x80x94Bixe2x80x94O (SBO) and Srxe2x80x94Taxe2x80x94Nbxe2x80x94O (STN) are adopted to take advantage of the spontaneous residual polarization of dielectric materials. If a ferroelectric material is used, however, resultant step coverage will be not so good. That is to say, it is usually difficult to form a ferroelectric thin film with a uniform thickness over the stepped portions.
As can be seen, no matter whether the high-dielectric-constant material or ferroelectric material is used, a technique of forming a thin film of either material over tiny stepped portions at a good coverage plays a key role to attain a desired capacity. As for the step coverage, a CVD process is superior to a sputtering technique, for example. This is why research and development has been carried on vigorously to form a film with good dielectric properties by a CVD process.
Examples of the CVD processes applicable to deposition of a BST or PZT film include a thermal CVD process and a plasma-enhanced CVD process. According to the thermal CVD process, organometallic complexes, containing the constituent metal elements of the film to be formed, are used as source materials. These organometallic complexes are dissolved in a solvent such as butyl acetate or tetrahydrofuran (THF), vaporized and then introduced into a reaction chamber, thereby causing a chemical reaction among them on a heated substrate. According to the plasma-enhanced CVD process, reaction of organometallic complexes on a substrate is accelerated by plasma generated within a reaction chamber. Also, in the thermal or plasma-enhanced CVD process, a plurality of source materials may be mixed at a predetermined ratio by various techniques. For example, according to a technique, respective solutions of organometallic complexes are mixed at the predetermined ratio and then vaporized. Another technique is vaporizing respective solutions of organometallic complexes and solvents and then mixing the resultant gases at the desired ratio. For instance, in depositing a BST film by a CVD process, three organometallic complexes Ba(DPM)2, Sr(DPM)2 and Ti(O-iPr)2(DPM)2 (where DPM is dipivaloylmethanato) are used as respective source materials, dissolved in a solvent such as butyl acetate at room temperature and then mixed at a predetermined weight ratio. Next, the mixture is introduced into, and vaporized by, a vaporizer that has been heated up to about 220xc2x0 C. Thereafter, these three organometallic complexes vaporized are introduced into a reaction chamber, in which a substrate has been heated up to about 400xc2x0 C. to about 700xc2x0 C. And then these three organometallic complexes vaporized are allowed to react with each other on the substrate, thereby forming a BST film thereon.
The organometallic complexes such as these are likely to combine with each other to form a copolymer, generally speaking. Accordingly, a variation in vaporization temperature or decomposition happens easily. Thus, in many cases, the formation of such a copolymer is prevented by a steric hindrance state, which has been created through the coordination of a so-called xe2x80x9cadductxe2x80x9d such as a tetraglyme group.
In recent years, a complementary MOS (CMOS) LSI including CMOS transistors has been further downsized, and a CMOS LSI with a design rule of 0.25 xcexcm has been used practically these days. An MOS transistor is a device with four terminals, i.e., gate, source, drain and semiconductor substrate. The gate electrode and the semiconductor substrate are electrically isolated from each other due to the existence of a gate insulating film therebetween. The potential at the gate electrode changes the quantity of carriers to be induced in a region of the semiconductor substrate just under the gate insulating film (i.e., channel region) and also changes the amount of drain current flowing. Based on this principle, the current flowing between the source and drain in the MOS transistor is controllable in terms of the value and ON/OFF states thereof.
In this case, no leakage current should flow between the gate electrode and any other terminal (i.e., source, drain or semiconductor substrate) in a single MOS transistor. Accordingly, the gate insulating film is required to exhibit very high insulation properties and reliability. For example, in an MOS transistor included in a CMOS LSI with a design rule of 0.5 xcexcm, the thickness of the gate insulating film is about 10 nm and the intensity of an electric field applied to the gate electrode during the operation of the transistor is 3 to 4 MV/cm. In this case, since the maximum rated electric field is about 8 MV/cm, the dielectric breakdown voltage of the gate insulating film should be about 10 MV/cm. Also, the gate insulating film needs to ensure good reliability for 10 years if the film is subjected to a TDDB test, for example.
To meet all of these severe requirements, a silicon dioxide film of quality has heretofore been used as gate insulating film for an MOS transistor. The silicon dioxide film is often formed by a so-called thermal oxidation process. Specifically, a silicon substrate is placed within an electric furnace and then heated up to 800 to 900xc2x0 C. with oxygen or water vapor introduced into the electric furnace, thereby growing a silicon dioxide film on the silicon substrate. The silicon dioxide film grown is sometimes densified within a non-oxidizing gas such as nitrogen gas by being heated up to the maximum temperature in all the process steps to be performed before the transistor is completed. The gate insulating film ensuring very high reliability has been formed in this manner.
Parts of a transistor that should be made of an insulating film of quality are not limited to the gate insulating film. For instance, a passivation film of silicon dioxide is also required to be no less reliable than the gate insulating film is. Specifically, the gate electrode of a transistor has been made of polycrystalline silicon (poly-Si) such that source/drain regions can be self-aligned with the gate electrode. In this case, the gate electrode is formed by patterning a polysilicon film and then the SiO2 passivation film is formed on the surface of the gate electrode to prevent leakage current from flowing between the surrounding portions of the gate electrode and the semiconductor substrate.
Furthermore, the same statement is equally applicable to various MOS devices other than an MOS transistor. For example, a capacitive insulating film, which is interposed between upper and lower electrodes of an MOS capacitor (i.e., the semiconductor substrate and polysilicon electrode), is also required to be as reliable as the gate insulating film. The same requirement is also imposed on a capacitive insulating film for a trench capacitor as an MOS device and on a capacitive insulating film that is formed on a storage node of a DRAM memory cell as an MIM capacitor.
It is known, however, that a large quantity of carbon compounds is left within a dielectric film that has been formed using the CVD source materials. The existence of carbon compounds in a dielectric film affects the electrical characteristics of a semiconductor device including the dielectric film. For example, the dielectric constant of the dielectric film decreases or leakage current increases due to the residual carbon compounds. Also, if the residual carbon compounds change into mobile ions, then the ions move along with the electric field resulting from the operation of capacitors and are segregated at an interface, thus possibly deteriorating the reliability of the dielectric film.
Carbon compounds left in a dielectric film can be removed by heating the dielectric film up to an elevated temperature of 800xc2x0 C. or more. However, if heat treatment is conducted at such a high temperature, then the characteristics of an MOS transistor that has already been formed on a substrate are affected. Accordingly, that high-temperature heat treatment is disadvantageous for the overall fabrication process, because the resultant characteristics of the semiconductor device are likely to deteriorate and because the processing efficiency is not so good.
Such an adverse phenomenon may occur not just in the dielectric film of a semiconductor device but also in any other components of the semiconductor device or components of any device other than the semiconductor device.
Also, the gate insulating film, passivation film or capacitive insulating film that is formed by thermal oxidation as in the prior art would not be able to cope with forthcoming further miniaturization or performance enhancement of MOS devices.
Among other things, while a silicon substrate or a polysilicon gate electrode formed on the silicon substrate is thermally oxidized, the profile of a dopant that has been introduced into the substrate or electrode changes as the substrate or electrode is held at an elevated temperature during the thermal oxidation. This is a serious problem, because the performance of the transistor deteriorates in that situation. In a thermal oxidation process, the temperature of a silicon substrate is generally kept as high as 800xc2x0 C. or more. At such an elevated temperature, a dopant that has been introduced into the silicon substrate diffuses. Thus, the as-designed dopant profile cannot be maintained and the performance of the transistor deteriorates. Also, if thermal oxidation is conducted after source/drain regions have been defined (e.g., when a capacitive insulating film is formed for a DRAM memory cell), then the dopant that has been introduced to form the source/drain regions diffuses, too. As a result, a shallow diffused layer cannot be formed, thereby constituting a great obstacle to the miniaturization of MOS devices. Furthermore, in a CMOS transistor of a dual-gate type, boron that has been doped into the gate electrode of a p-channel MOS transistor diffuses from the gate electrode into the semiconductor substrate or gate insulating film as a result of heat treatment, thus also deteriorating the performance of the transistor.
According to the prior art thermal oxidation technique, it is difficult to form a silicon dioxide film exhibiting good insulation properties at a low temperature of 600xc2x0 C. or less. This is because the oxidation rate of silicon is very low in this temperature range. To increase the oxidation rate, the pressure of water vapor may be raised. By adopting this idea, a technique of carrying out thermal oxidation at 10 atmospheric pressure or less was proposed (see Tsubouchi et al., Materials for Meeting of Technical Group on Electronic Part Materials No. CPM-79-36 (1979), Institute of Electronic, Information and Communication Engineers of Japan). In this pressure range, however, a practical oxidation rate is not attainable at 600xc2x0 C. or less.
In some cases, a silicon nitride or silicon oxynitride film is used instead of a silicon dioxide film. But similar unwanted phenomenon occurs because the temperature of a substrate should also be kept high in the process step of nitriding silicon as in the oxidation process step.
A primary object of the present invention is providing (1) a method for efficiently removing foreign matter from inside a substance, which is applicable to the formation of a dielectric film with excellent properties and (2) a method and apparatus for forming a film using this foreign matter removing method.
Another object of the present invention is providing a method of forming a film for a high-performance semiconductor device by forming an insulating film of quality at a temperature low enough to prevent the diffusion of a dopant introduced into a semiconductor substrate and by eliminating the high-temperature process as much as possible.
According to an inventive method for removing foreign matter from an object to be processed, the foreign matter is removed by exposing the object to a fluid, which dissolves the foreign matter, while keeping the fluid in a supercritical or subcritical state.
In general, the ability of a fluid to dissolve foreign matter greatly improves in its supercritical or subcritical state. According to the method of the present invention, the foreign matter can be removed from inside an object by taking advantage of this action of the fluid. In this case, the fluid can enter the supercritical or subcritical state at a relatively low temperature, generally speaking. Thus, the foreign matter can be removed easily even if the object is not heated to an excessively high temperature. That is to say, foreign matter can be removed from even an object, which usually deteriorates at an elevated temperature, without degrading the quality of the object. In addition, since excellent processing efficiency is attainable, it takes just a short time to remove the foreign matter.
In one embodiment of the present invention, the object may be made of a first substance that has been produced as a result of a reaction of a source material containing an organic metal or an organometallic complex. And the foreign matter may be a second substance made of a carbon compound that has also been produced as a result of the reaction of the source material containing the organic metal. In such an embodiment, the carbon compound is removable easily even if the object is not heated to an excessively high temperature. An object made of a substance that has been produced as a result of a reaction of a source material containing organic metals or organometallic complexes (usually, an MOCVD process) contains a particularly large quantity of carbon compounds. Even so, the carbon compounds are removable easily without deteriorating the quality thereof, because the object is not heated too much.
In this particular embodiment, the object may be a film that has been formed with the source material containing the organic metal or the organometallic complex dissolved in a solvent thereof. Although the carbon compounds are very likely to be left in such a case, the carbon compounds are still removable efficiently.
Specifically, the solvent of the source material is preferably at least one compound selected from the group consisting of hydrocarbon and halogenated hydrocarbon compounds. Even though the hydrocarbon or halogenated hydrocarbon compound is very likely to be left in the film in such a case, the hydrocarbon or halogenated hydrocarbon compound is still removable efficiently.
More particularly, the source material may be a compound containing a dipivaloylmethanato (DPM) group. In such a case, a film made of a high-dielectric-constant material such as BST is formed and a lot of carbon compounds, which constitute methoxy groups, for example, are left in the film. Even so, the carbon compounds are easily removable efficiently.
In another embodiment, carbon dioxide may be used as the fluid. Foreign materials like carbon compounds are highly soluble in carbon dioxide in the supercritical or subcritical state. Thus, the foreign matter removal ability can reach an amazingly high level in such an embodiment.
A first exemplary film forming method according to the present invention is adapted to form a film on a substrate using a source material, which produces first and second substances when reacted, such that the film is made of the first substance. The method includes the steps of: a) preparing the source material and a solvent that dissolves the second substance; b) keeping the temperature and pressure of the solvent in a supercritical or subcritical state; and c) heating the substrate such that the surface of the substrate is kept at such a temperature as allowing the source material to react, while making the source material and the solvent in the supercritical or subcritical state come into contact with the surface of the substrate such that a film is formed on the substrate out of the first substance and that the second substance is dissolved in the solvent and removed.
According to this method, a film can be formed out of the first substance, while at the same time, the second substance, which is going to enter the film, can be blocked and dissolved in the solvent. This is because the ability of the solvent greatly improves in its supercritical or subcritical state. Thus, a film can be formed out of only the first substance without performing any additional process step of removing the second substance after that.
In one embodiment of the present invention, the solvent preferably dissolves the source material in the supercritical or subcritical state.
In another embodiment of the present invention, a compound containing an organic metal or an organometallic complex may be prepared in the step a) as the source material. A film can be formed at a low temperature in such a case. Thus, even if a component, which usually exhibits inferior performance at an elevated temperature, is provided on a substrate, such decline in performance is suppressible.
In this particular embodiment, a compound containing a dipivaloylmethanato (DPM) group may be prepared in the step a) as the compound containing the organic metal or the organometallic complex. In such a case, a BST film, which shows a high dielectric constant but to which carbon compounds are easily mixed, is formed. Thus, the present invention is applicable particularly effectively to such a situation.
In still another embodiment, carbon dioxide is preferably prepared in the step a) as the solvent.
In yet another embodiment, a film selected from the group consisting of paraelectric, ferroelectric and metal films may be formed in the step c) as the film made of the first substance.
Specifically, a film selected from the group consisting of crystallized paraelectric, ferroelectric and metal films may be formed in the step c).
In still another embodiment, the substrate may include at least one film selected from the group consisting of paraelectric, ferroelectric and metal films. Then, the first film forming method of the present invention is applicable to fabricating an MFISFET or MFMISFET.
Alternatively, a semiconductor substrate may be used as the substrate. Then, the first film forming method of the present invention is applicable to fabricating an MISFET.
In still another embodiment, a dielectric or metal film in a crystallized state may be formed in the step c) as the film made of the first substance. In such an embodiment, a ferroelectric film with excellent crystallinity can be formed thereon.
An inventive semiconductor device is of an MIS type including a ferroelectric layer and a gate electrode over a semiconductor substrate. The ferroelectric layer has been formed by making a source material, which produces first and second substances when reacted, and a solvent, which dissolves the second substance, come into contact with the surface of the semiconductor substrate, while keeping the temperature and pressure of the solvent in a supercritical or subcritical state.
In this manner, a ferroelectric layer with excellent crystallinity and a great degree of residual polarization, i.e., a ferroelectric film showing much less varied residual polarization among the lots, can be formed. Accordingly, if such a ferroelectric layer is formed in a nonvolatile memory device, then information can be retained a lot more accurately in the device.
In one embodiment of the present invention, the device may further include a paraelectric layer between the ferroelectric layer and the semiconductor substrate. The paraelectric layer has been formed by making the source material, which produces the first and second substances when reacted, and the solvent, which dissolves the second substance, come into contact with the surface of the semiconductor substrate, while keeping the temperature and pressure of the solvent in the supercritical or subcritical state. In such an embodiment, a crystalline paraelectric layer can be formed and the crystallinity of the ferroelectric layer that is formed on the paraelectric layer can be further improved.
A second exemplary film forming method according to the present invention is adapted to form a film on the surface of an object to be processed. The object is exposed to a fluid kept in a supercritical or subcritical state, thereby forming a film out of a reactant, which has been produced as a result of a reaction between a substance contained in the object and a substance contained in the fluid, on the surface of the object.
According to this method, while the fluid is in the supercritical or subcritical state, the surface of the object can be exposed to a substance at a high concentration, which shows high reactivity with the object when vaporized. Thus, a thin film can be formed efficiently. In this case, the fluid can enter the supercritical or subcritical state at a relatively low temperature, generally speaking. Thus, the film can be formed even if the object is not heated to an excessively high temperature. That is to say, the film can be formed on even an object usually exhibiting inferior properties at an elevated temperature without degrading the quality of the object. In addition, since excellent processing efficiency is attainable by appropriately controlling the conditions in the supercritical state, it takes just a short time to form the film.
For example, water in a supercritical state, which is kept at its critical temperature (374xc2x0 C.) or more and at its critical pressure (22.04 MPa=217.6 atm) or more, functions as an oxidizing agent by itself and dissolves a lot of oxidizing agent like surrounding oxygen. Thus, it is known that a lot of substance can be burned, or oxidized, within that water in the supercritical state, theoretically speaking. If the surface of an Si substrate is exposed to that supercritical water, then a lot more oxidizing agent can be supplied onto the surface of the Si substrate compared to the conventional thermal oxidation technique. Accordingly, a silicon dioxide film can be formed at a temperature as low as 600xc2x0 C. or less. Also, the silicon dioxide film formed this way has been oxidized by a different mechanism than a native oxide film, which is porous and has low dielectric strength. Thus, the dielectric strength of the silicon dioxide film is superior.
In one embodiment of the present invention, a substance containing oxygen may be used as the fluid, and an oxide film of the substance contained in the object may be formed on the surface of the object.
In this particular embodiment, the fluid may be at least one substance selected from the group consisting of water, oxygen and nitrous oxide.
In another embodiment, a silicon dioxide film may be formed on the surface of the object using a silicon layer as the object.
In an alternative embodiment, a silicon oxynitride film may be formed on the surface of the object using a silicon nitride layer as the object.
In another alternative embodiment, the object on which the oxide film has been formed may be further exposed to a fluid containing nitrogen in a supercritical or subcritical state, thereby changing the oxide film into an oxynitride film.
In still another embodiment, not only at least one of water, oxygen and nitrous oxide but also an oxidation accelerator may be used as the fluid. In such an embodiment, a favorable oxidation-etching balance can be maintained such that oxidation prevails over etching.
In this particular embodiment, at least one of ozone (O3), hydrogen peroxide (H2O2), nitrogen dioxide (NO2) and nitrogen monoxide (NO) may be used as the oxidation accelerator.
Alternatively, a substance containing chlorine, as well as the substance containing oxygen, may be used as the fluid. In such an embodiment, a favorable oxidation-etching balance can also be maintained such that oxidation prevails over etching.
In this particular embodiment, the substance containing chlorine may be at least one substance selected from the group consisting of hydrogen chloride, chlorine, sodium chloride, potassium chloride, calcium chloride and other metal chlorides.
In an alternate embodiment, a substance producing alkali metal ions, as well as the substance containing oxygen, may be used as the fluid. In such an embodiment, a favorable oxidation-etching balance can also be maintained such that oxidation prevails over etching.
In still another embodiment, a substance containing nitrogen may be used as the fluid, and a nitride film of the substance contained in the object may be formed on the surface of the object.
Specifically, the fluid may be at least one substance selected from the group consisting of nitrogen, ammonium and an amine.
Alternatively, a silicon nitride film may be formed on the surface of the object using a silicon layer as the object.
An inventive film forming apparatus is adapted to form, on a substrate, a film out of a first substance that has been produced as a result of a reaction of a source material containing an organic metal or an organometallic complex. The apparatus includes: a vessel for placing the substrate therein; a container for storing the source material and a solvent that dissolves a second substance produced as a result of the reaction of the source material; a temperature/pressure regulator for regulating the temperature and pressure of the solvent to keep the solvent in a supercritical or subcritical state; a source material feeder for supplying the source material into the vessel; a source material concentration controller for controlling the concentration of the source material by dissolving the source material that has been supplied from the source material feeder in the solvent; and a substrate temperature control mechanism for keeping the surface of the substrate at such a temperature as allowing the source material to react.
The film forming apparatus can form a film out of the first substance while eliminating the mixture of unwanted substance in the above-described way.